Frost Dates for 55808 — Duluth, Minnesota

Half of springs at Superior, WI see their last 32°F night by May 6; only one in ten is past it by April 24, and one in ten still has one after May 25. Planting tender crops after May 15 leaves a 30% risk of a frost on them, after May 19 20%, and after May 25 10%. The 19 days between the even-odds date and the one-in-ten date is wider than at 98% of the 142 Minnesota stations behind this site's ZIP pages.

In fall the first 32°F night comes by October 9 in half of years, as early as September 24 one year in ten, and holds off past October 24 one year in ten. A hard freeze of 28°F follows 15 days later, around October 24; in spring the one-in-ten date for 28°F is May 9, 16 days before the 32°F one. The freeze-free season lasts 154 days in a typical year, longer than at 61% of the 142 Minnesota stations; it reaches 175 days one year in ten and falls short of 132 one year in ten. On average 161 nights a year drop to 32°F or below. Nights of 36°F, when frost can still settle on leaves in low, open spots, continue 15 days longer, to about May 21, and return around September 28. Severe cold of 16°F or below comes every winter, usually between November 18 and March 29.

July is the warmest month at Superior, WI, averaging 76°F by day and 58°F at night; January is the coldest at 23°F and 6°F. A freezing night is all but certain in every month from October to April (a 90% chance or more). From June to August the chance of a 32°F night stays under 10% each month. In spring, May still brings one in 72% of years.

Warmth for growing adds up to 1,706 growing degree days a year (base 50°F), fewer than at 92% of the 142 Minnesota stations. 21% of them (353) have built up by the end of June, and July adds the most, 529. Precipitation totals 31.5 inches a year, most in June (4.6") and least in January (0.8"); May to September bring 61% of it.

The nearest other station, Duluth Harbor Stn, MN, 9 miles northeast of Duluth, has a last frost 8 days later (May 14), a first frost 3 days later (October 12), a season 5 days shorter.

The first measurable snow (0.1 inch or more) in Duluth usually arrives around November 16, and the last of the season around April 9. Those are the middle dates across the winters of 1991–2020 at Superior, WI, 8.2 miles away: half of years saw snow earlier, half later. Every one of those winters saw snow. The last 32°F night of spring typically comes 27 days after the last snow.

ZIP 55808 is 8 miles from Superior, WI, which stands at 607 feet, the nearest NOAA station that publishes freeze probabilities for 1991–2020. 1 other ZIP code shares the station; its figures are presented in full for 54880 (Superior). The 2023 USDA Plant Hardiness Zone Map lists ZIP 55808 in zone 4b, the band where the coldest night of the year averaged -25 to -20°F (-31.7 to -28.9°C) over 1991–2020; a zone describes winter lows, not frost dates.